Electric contact



Sept. 10, 1935. K. G. soDERBERG 2,013,868

ELECTRIC CONTACT Filed Dec. 17, 1930 ffatented Sept. 10, 1935 ELECTRIC CONTACT l Gustaf Soderberg, Detroit, Mich., assignor to Udylite Process Comp poration of Michigan any, Detroit, Mich., a cor- Applicafion December 17', 1930, serial No. soaaos 3 Claims.

This invention relates to electric contacts.

It is well known that when two electrical conductors of copper, or any other suitable metal, are held together mechanically that a certain 5 restrictive resistance to the current fiow occurs at the joint which is called the contact resistance. This contact resistance effects a heating of the contact surfaces which expedites the oxidation of these surfaces. This contact resistance increases with the oxidation of the contact surfaces of the conductors. It is old to obviate to a substantial p degree this increase in contact resistance by coating or plating the contact surface with a metal such as cadmium that does not readily corrode.

This invention is concerned with the eliminating of this contact resistance between the contacting surfaces of two electrical conductors and more specifically with such contacts as have been coated with a dissimilar metal for the purpose of increasing the corrosion resistance and/or decreasing the contact resistance. By experiment I have made the surprising discovery' that what has heretofore been known as the contact re'- sistance actually comprisesztwo component resistances, namely, that occurring between the two surfaces which are mechanically held in contact and that occurring at the junction of the base. metal with the coating metal. This latter resistance I shall term the film resistance.

It is the object of this invention to decrease the contact resistance between electric contacts by treating the contacts chemically and/or thermally so that the union of the base metal and the coating metal will be as complete and intimate as possible.

In the drawing:

Fig. 1 shows an ordinary type of fuse mounted in fuse clips.

Fig. 2 is a diagraminatic illustration showing the different resistances in an electrical contact which is simply coated or plated with a metal forthe purpose of increasing the corrosion resistance and/or decreasing contact resistance.

As shown in Fig. 1 the fuse is of the ordinary and well-known type provided with copper or any other suitable metal end ferrules 2 and 3. The end ferrules are arranged to be received in the copper or brass fuse clips 4 and 5 which are suitably mounted upon the base 6. The fuse and fuse clips are preferably coated with cadmium owing to its high corrosion resisting properties and contact resistance reducing properties. The fuse shown in Fig. 1 is herein described merely 65 for the purpose of exemplifying one of the numerous electric contacts which may be subjected to the hereinafter disclosed method for decreasing contact resistance.

In Fig. 2 the base metal may be referenced A. This base metal can be any well-known electrical 5 conductor such as copper which is usually used owing to its cheapness and its low ohmic resistance. The base metal may be coated or plated in any of the well-known ways with any suitable metal C which has the property of in- 10 creasing the corrosion resistance and/or decreasing the contact resistance, such as cadmium. D represents the contact surfaces with the contact resistance. B designates the junction between the base metal A and the coating C and also the re- 15 sistance to the current flow which occurs at the junction. Since the resistance D which occurs between the contacting surfaces of the coatings C of the base metal A, is known as the contactv resistance, for purposes of description the r :33 sistance which occurs at the junction of the coating C and the base metal A will hereinafter be termed the film resistance. Hence, the total resistance to the current flow through the contact i is made up bythe contact resistance D plus the 2;,

film resistance B.

It is the object of this invention to obviate and reduce to a negligible degree the'film resistance B. This is achieved in three ways, namely, by chemical treatment of thepbase metal A prior to 30 coating or plating the same; by thermally treating the coating metal C and base metal A at a temperature too low to cause any appreciable and visible oxidation of the coating; by chem-V ically treating the base metal A prior to the coatfg; ing or plating of the same and after the plating of the base metal A with the metal C thermally treating the coating and base metal.

I have found that this film resistance can be removed practically completely by chemically 40 treating the base metal A prior to the application of the coating C so that the surface of the base metal is made chemically clean and the active atoms of the base metal so exposed that the coating C connects with the base metal as inti- 45 mately as possible. The composition and nature of the chemical reagent which is to be used obviously should vary with the nature and composition of the metal. I have found vthat liquids containing nitric acid produce the desired results 50 on copper and many of its alloys such as the brasses and bronzes. In case the contact is aluminum I have found that the use of hydrofluoric acid is effective. The point is, that any chemical reagent can be used which will render the surface of the base metal which is to be' coated chemically clean and permit the atoms of the base metal to intimately contact with the atoms of the plated or coating metal. In other words, any chemical reagent can be used which will dissolve and activate the base metal, whether the base metal be copper, aluminum, or any other suitable metal. By base metal, as used herein, we mean the metal which receives the coating. Hence, in case several coatings are applied, one upon the other, then as herein used, base metal would also include any coating which had another coating superimposed thereupon. In this case there are two or more film resistances to be overcome.

After the metallic coating has been applied to the base metal A the junction between the two is heated to such a temperature and for such a period of time as will cause a diffusion of one metal into the other or of both metals each into the other. The range of temperature of this thermal treatment should be low enough and for a period of time insufficient to cause any visible oxidation of the coating. Of course, if the temperature of the heat treatment is increased the period of time required is decreased and vice versa. In case a cadmium coating is used, then the heat treatment should be at a temperature not higher than 250 C. since cadmium starts to oxidize at about 250 C. If the heat treatment is eected at a temperature and over a sufficient period of time to voxidize the coating, then the oxide should -be removed in any suitable way to expose the metallic coating of the contacting surfaces, otherwise the oxides will cause an increase in the contact resistance. It is understood that while the chemical treatment of the base metal prior to the application of the v metal coating and thermal treatment is very advantageous, this chemical treatment can be omitted and the coated base metal merely thermally treated.

Since the heat treatment causes a diffusion of one metal into the other or of both metals one into the other, one would naturally expect an increase in total contact resistance since it is wellknown that an alloy always has a higher electrical resistance than that of any of its component metals. The surprise was that instead of obtaining an increase of total contact resistance an actual decrease of total contact resistance was obtained. Hence, this shows that even though a small thickness of alloy film was formed the resistance of the alloy film thus produced was very much smaller than the resistance which was made to disappear, this resistance being called the film resistance.

t is understood that the method herein disclosed can'be used wherever the base metal of the contact is coated with a different kind of metal, preferably one of higher corrosion resistance and contact resistance reducing properties, but 'for purposes of description only, I shall set forth a specific example wherein the contact comprises a base metal of copper and a corrosion resisting coating of cadmium. In such a contact I found that where the copper was chemically treated with a 20% by volume nitric acid solution only, that the contact resistance at 200 lbs. pressure was .0000045 ohms, at 500 lbs. pressure 000002 ohms. I also obtained substantially the same results where the copper vcontact had been cadmium plated and heated to 210 C. for one hour. Where the copper contact was plated with cadmium and not subjected to. heat treatment after the application of the cadmium coating, the resistance of the contact at a pressure of 200 lbs. per square inch was .0000120 ohms and at 500 lbs. per square inch pressure .0000053 ohms. Hence, by treating the contact according to the method above disclosed the film resistance is substantially removed and consequently the contact resistance is reduced to substantially one-third of what it would be without treatment according to the above disclosed method.

From the above description it is evident that there is here produced an invention for eliminating the resistance to current flow through an eleco tric contact comprising a -base metal which is coated with a Vdissimilar metal having the properties of increasing the corrosion resistance and/or s decreasing the contact resistance which is essentially characterized by the removal of the resistance to current fiow at the junction of the base metal and the coating metal by chemical and/or thermal treatment of one or both of the said metals.

I claim:

1. The method of reducing resistance to current flow in an electric contact comprising applying a coating of a corrosion resisting metal to the base metal of the contact at a temperature below the alloying temperatures of said metals resulting in the formation of a film resistance at the line of juncture between the said metals, then heating this product to a temperature at which the said metals alloy with each other to destroy or modify this film resistance.

2. The method of reducing resistance to current fiow inan electric contact ,comprising electrodepositing a coating of a corrosion resisting metal on the base metal of the contact at a'temperature below the alloying temperatures of the said metals whereby a film resistance is formed at the juncture between the coating and the base metal, and then heat treating the base metal and electrodeposited coating of corrosion resisting metal at a temperature just sufiicient to rer move the vfilm resistance and form a thin alloy at the juncture between the metals but not of sufficient intensity nor long duration to form an objectionable oxide coating.

3. The method of reducing resistance to current .fiow in an electric contact comprising electrodepositing cadmium from an aqueous solution on the base metal of the contact whereby a film resistance is formed at the juncture between the cadmium and the base metal, and then heat treating the base metal and electrodeposited cadmium coating at a temperature sufficient to remove the film resistance and form a thin alloy at the juncture between the metals but not of sufficient intensity nor long duration to form an objectionable KARL GUSTAF SODERBERG. 

